In the past, aluminum and its alloys have been the materials of choice for dual damascene interconnect metallurgies and other Back-End-Of-Line (BEOL) metallization structures utilized in semiconductor devices. These interconnects have typically been embedded in dielectric materials (e.g., SiO2) known as interlayer dielectrics (ILD). The delay in the BEOL structure, and hence its performance characteristics, are dictated by the product of the resistance of the interconnect and the capacitance provided by the ILD materials.
The physical limitations of aluminum interconnects, and in particular their relatively high resistivities, have prevented them from accommodating the need for increased circuit densities and speeds in semiconductor devices. Consequently, copper-based metallurgies have evolved as replacements for aluminum metallurgies in dual damascene interconnect metallurgies and other BEOL metallization structures. Copper-based interconnect metallurgies offer lower resistivities and comparatively lower susceptibility to electromigration failure compared to their aluminum counterparts. The use of copper-based metallurgies also allows capacitance to be exploited in the optimization of interconnect performance. To this end, ultra-low dielectric constant materials have begun supplanting SiO2 as the dielectric materials of choice in which metal lines are embedded.
The material properties of the ILD in dual damascene applications are essential for integration of low RC (Resistance/Capacitance) backend solutions. In particular, this dielectric material must be mechanically strong and possess a very low dielectric constant or k-value. In practice, this dielectric constant must be much lower than that of SiO2, which has a dielectric constant of about 4.1.
One of the challenges facing the commercial implementation of dual damascene processes is the lack of an optimal dielectric material for use in the ILD. In particular, most existing dielectric materials, such as silicon dioxide, porous hydrogen and carbon containing glasses (e.g., SiCOH), or dielectrics based on aromatic hydrocarbon thermosetting polymers, are either unsuitable for use in the ILD layer, are fraught with mechanical stability issues or exhibit k-value integrity issues during processing, or simply have too high of a k-value. Air gap ILDs possess potentially the best k-value, but come with significant patterning, processing and mechanical integrity challenges.
Other known low-k materials are unsuitable for use in dual damascene processes because they require that a protective layer be formed on the etched dielectric layer in order to protect the dielectric material during processing. For example, porous low-k materials require pore sealing for all of the processes used to etch them (i.e., for via formation and cleaning), as well as for the metallization processes used to form the metal interconnects. Potentially, each processing step (e.g., etching, post-etch treatment to remove etch residues, the preparation of exposed metal surfaces, and metal deposition) can degrade the BEOL dielectric constant. Silicon and oxygen-based low-k films whose low k-value is enabled by the presence of carbon are particularly prone to k-value degradation in this regard, since the only known processes that can adequately etch them for high performance and high density computing applications will themselves interact with the carbon and hydrogen in the films.
The use of boron-nitride (BN), and in particular, boron nitride nanotubes (BNNTs), as ILD materials has also been investigated. These materials have attractive hardness and can theoretically provide dielectric layers with very low K-values. However, BNNT compositions have a number of fundamental flaws that have prevented their successful use in BEOL applications. In particular, the k- values achieved with dielectric layers formed from these materials are found to be significantly higher than the values that are theoretically possible, and the mechanical properties of the resulting layers have been found to be inadequate. Moreover, BNNT layers are typically grown from catalyst layers. The extra steps involved in patterning (masking and etching) the catalyst at all the levels of metal and dielectric make this process prohibitively expensive.
There is thus a need in the art for a low-k dielectric material that is suitable for use in dual damascene processes and that exhibits good mechanical properties. There is also a need in the art for a dielectric material whose use in a damascene process does not require additional processing steps, such as those required to manage the impact of film porosity or to pattern catalyst layers. There is further a need in the art for dielectric materials whose k-values are resistant to change from processes that are commonly used in BEOL processes. These and other needs are met by the processes and materials disclosed herein and hereinafter described.